SAFETY IN THE PLANT


According to Brazier, regular “up-close- and-personal” inspection is critical given that internal damage and material accumulation is typically not visible at a distance. In many cases, this set-it-and-forget-it


approach is owing to a less-than-complete understanding of the risk factors associated with composite vents compared to more modern alternatives.


COMPOSITE VENTS Te composite vent design was introduced by BS&B Safety Systems more than 50 years ago and marked a milestone in the achievement of low set pressure from relatively thick sheet metal. Composite vents are typically made of a


three-part sandwich construction of plastic film seal between two sheets of metal, usually stainless steel. Te two metal sections are slotted with intermittent uncut sections that control the burst pressure and vent opening. Te outlet side metal section controls


burst pressure, while the inlet metal section protects the fragile plastic film and provides some degree of vacuum resistance. In this design, the vents “open” typically at 1 to 1.5 PSI set pressure. Composite vents can be either flat or domed, depending upon the application conditions, with domed vents being recommended for vacuum service. Te disadvantage of this type of


construction is that it allows dust to penetrate between the inlet side sheet metal and plastic film, resulting in leakage over time when angular dust particles puncture the seal.


Although significant damage to explosion


vents may be visible at a distance, internal damage may not be observable without close inspection. If damaged, the vent could draw in humid air and even rainwater, particularly under light vacuum pressure. Tis can potentially affect the product inside process equipment. For dry items in storage, for example, moisture can cause clumping of material and even the agglomeration of dusty material. “It can be a real inconvenience if the


customer’s process is impacted because an explosion vent is damaged,” says Brazier. In addition to being unsanitary,


particularly for food processors, the increase in mass due to the accumulation of material means that the vent will not open as fast as planned. Te result can be higher-than- expected pressure in the dust collector or other protected equipment in the event


of explosion vent activation. “Without the expected relief [from venting] a dust explosion can overpressurise the equipment, causing significant damage,” says Brazier. “Worst case, the explosion could escape through a weak point – and not the explosion vent – causing an uncontrolled release of flame, dust and pressure where it wasn’t planned.”


As the demand for explosion protection technology grew, BS&B later developed the single section vent. In this design, a single sheet of metal is intermittently cut about its perimeter and then gaskets are applied to cover the cut pattern. Eliminating the fragile plastic film seal and one sheet of metal greatly reduces the mass of an explosion vent, making it more efficient at responding to the rapid rate of pressure rise arising from a dust or gas explosion.


SINGLE SECTION COMPOUND DOMES In 2010, BS&B improved on the single section vent design by altering the shape of the dome to a novel compound geometry. Tis advance was born out of a desire to increase the vacuum resistance of the vents, without increasing the weight. Te compound dome has curves at the corners of rectangular and square vents that deliver even greater rigidity for high vacuum or vibration applications. Te primary purpose of the domed


construction, on the other hand, is to provide greater intrinsic vacuum resistance. Tis is important, as storage, pneumatic conveying and dust collection systems often operate under light vacuum conditions. Industrial dust collectors can also use short pulses of compressed air to clean and extend the life of their bag or cartridge filters. In these types of operating conditions,


domed vents are more stable and resilient, increasing service life when compared to flat alternatives. According to Brazier, the vent mass is critical when calculating vent sizing. Recent revisions of NFPA 68 include a review of vent mass for every application. Te results may mandate a larger vent area for less efficient, heavier explosion vents. In short, this means a composite vent


would require a much larger vent area to perform the same job than more efficient, lightweight and modern alternatives. Tis adds to the costs of installation. Te compound dome feature has also resolved another longstanding challenge


Centre opening traditional composite vent in mounting frame


involving the framing on which the vents are mounted. In many cases, these frames are built by the end users out of angle- iron. Over time, the framework can bend or buckle – a common occurrence that can reduce vacuum resistance and can even cause the vent to fail. Te improved compound dome design is more tolerant to variations in the framing and less sensitive to the operating application and installation conditions. For its flagship VSM product, the company has also included an integral frame to the vent flange to further simplify installation. “Tese design features provide more control over how the vent behaves in the field and much less relevance to what happens on the user side [installation],” says Brazier.


Although single piece vents offer the best solution for most applications, there are some exceptions where composite vent technology provides the best performance. In particular, this is for applications that use combustion systems with hot exhaust gases.


Te BS&B HTV vent has internal thermal insulation that allows for service conditions in excess of 1,000°F. With this type of vent, insulation material isolates the plastic film and burst control metal section from extremes of service temperature. “By placing the insulation inside


the layers of the composite vent, you can achieve very high resistance to temperature while maintaining the low set pressure explosion protection capabilities,” says Brazier.


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